The present invention relates to a luminescence device for use in a display apparatus, an illumination apparatus, etc., and more particularly to a luminescence device having a high electroconductivity due to a self-alignment characteristic of a liquid crystal compound.
An organic electroluminescence device (hereinafter sometimes called an “organic EL device”), as a type of luminescence device, has been an object of extensive research and development in the search for a high speed response and high efficiency luminescence device.
FIGS. 1 and 2 are schematic sectional views, each illustrating a basic structure of an example of an organic EL device. Referring to FIGS. 1 and 2, such organic EL devices include: a transparent substrate 1, a transparent electrode 2, a metal electrode 3, organic layer(s) 4, a luminescence layer 5, a holetransporting layer 6 and an electron-transporting layer 7. Such a basic structure of an organic EL device is disclosed in, e.g., Macromol. Symp., vol. 125, pp. 1-48 (1977).
As shown in FIGS. 1 and 2, an organic EL device generally has a structure including a laminate comprising a plurality of organic layers 4 sandwiched between a transparent electrode 2 and a metal electrode 3 and disposed on a transparent substrate 1.
In the structure of FIG. 1, the organic layers 4 comprise a luminescence layer 5 and a hole-transporting layer 6. The transparent electrode 2 comprises a material, such as ITO (indium tin oxide) having a large work function, so as to ensure a good hole injection characteristic from the transparent electrode 2 to the hole-transporting layer 6. The metal electrode 3 comprises a metal material having a small work function, such as aluminum, magnesium or alloys of these, so as to ensure a good election injection characteristic to the organic layers 4. The electrodes 2 and 3 may be formed in a thickness of 50-200 nm.
The luminescence layer 5 may, for example, comprise an aluminum quinolynol complex (a representative example of which is “Alq” having a structure as shown below) showing an electron-transporting characteristic and a luminescence characteristic. The hole-transporting layer 6 may comprise a material showing an electro-donating characteristic, such as triphenyldiamine derivatives (a representative example of which is “α-NPD” having a structure as shown below): 
The organic EL device of FIG. 1 shows a rectifying characteristic, and when a voltage is applied between the metal electrode 3 as a cathode and the transparent electrode 2 as an anode, electrons are injected from the metal electrode 3 into the luminescence layer 5, and holes are injected from the transparent electrode 2. The holes and electrons injected to the luminescence layer 5 are recombined in the luminescence layer 5 to form excitons, which cause luminescence. In this instance, the hole-transporting layer 6 functions as a layer for blocking electrons to provide a higher recombination efficiency at the luminescence layer/hole-transporting layer boundary, thereby providing an enhanced luminescence efficiency.
Further, in the structure of FIG. 1, an electron-transporting layer 7 is disposed between the metal electrode 3 and the luminescence layer 5 in the structure of FIG. 1. According to this structure, the luminescence function is separated from the functions of both electron transportation and hole transportation to provide a structure exhibiting more effective carrier blocking, thus realizing more efficient luminescence. The electron-transporting layer 7 may comprise, e.g., an oxadiazole derivative.
The organic layers 4 comprise two or three organic layers and may have a thickness of 50-500 nm in total of the two or three layers.
In the above-mentioned organic EL devices, the luminescence performance is critically determined by performance of injection of electrons and/or holes from the electrodes. The above-mentioned amorphous materials, such as Alq and α-NPD are not believed to provide sufficient carrier injection performances in view of the resultant electrode-organic layer boundaries.
Further, in the course of the recent development of organic EL devices, devices utilizing phosphorescence from a triplet exciton instead of fluorescence from a single exciton have been studied. This is discussed in, e.g., the following articles:    (1) “Improved Energy Transfer in Electro-Phosphorescent Device” (D. F. O'Brien, et al., Appl. Phys. Lett., vol. 74, no. 3, p. 422 (1999)); and    (2) “Very High-Efficiency Green Organic Light-Emitting Devices Based on Electro-Phosphorescence” (M. A. Baldo, et al., Appl. Phys. Lett., vol. 75, no. 1, p. 4 (1999)).
In the above articles, a structure including organic layers 4 of four layers as shown in FIG. 3 is principally used. More specifically, the organic layers 4 sequentially include, from the anode side, a hole-transporting layer 6, a luminescence layer 5, an exciton diffusion prevention layer 9 and an electron-transporting layer. As materials constituting the organic layers 4, in addition to Alq and α-NPD mentioned above, there have been enumerated carrier-transporting materials, such as CBP (4,4′-N,N′-dicarbazole-biphenyl) and BPC (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline), and phosphorescent compounds, such as PtOEP (platinum-octaethylporphyrin complex) and Ir(ppy)3 (iridium-phenylpyrimidine complex), respectively represented by the following structural formulae: 
The above-mentioned articles have reported structures as exhibiting a high efficiency, including a hole-transporting layer 6 comprising α-NPD, an electron-transporting layer 7 comprising Alq, an exciton diffusion-prevention layer 9 comprising BCP, and a luminescence layer 5 comprising CBP as a host and 6 mol. % of PtOEP or 6 wt. % of Ir(ppy)3 as a phosphorescent compound.
A phosphorescent compound has been particularly noted because it is expected to exhibit a high luminescence efficiency in principle. More specifically, excitons formed by carrier recombination comprise singlet excitons and triplet excitons in a probability ratio of 1:3. Conventional organic EL devices have utilized fluorescence emitted by transition from a singlet exciton to the ground state as available luminescence, but the luminescence efficiency thereof is limited to at most ca. 25% of generated excitons in principle. On the other hand, if phosphorescence of excitons generated from triplets is utilized, an efficiency of at least three times is expected, and even an efficiency of 100%, i.e., four times, can be expected in principle, if a transition owing to intersystem crossing between singlet and triplet states is taken into account.
Devices utilizing luminescence from triplet states are also proposed in JP-A 11-329739 (entitled “Organic EL Device and Process for Production Thereof”), JP-A 11-256148 (entitled “Luminescence Material and Organic EL Device Using Same”), and JP-A 8-319482 (entitled “Organic Electroluminescence Device”, corresponding to U.S. Pat. Nos. 5,698,858 and 5,756,224).
In the case where the luminescence layer comprises a host material having a carrier-transporting function and a phosphorescent guest material, a process of phosphorescence attributable to triplet excitons may include unit processes as follows:                (1) transportation of electrons and holes within a luminescence layer,        (2) formation of host excitons,        (3) excitation energy transfer between host molecules,        (4) excitation energy transfer between the host to the guest,        (5) formation of guest triplet excitons, and        (5) phosphorescence.        
Energy transfer in each unit process is caused by competition between various energy transfer and deactivation processes. For example, a host exciton, even if formed, can lose its energy by nonradiative deactivation prior to energy transfer to another host. Also in the process of energy transfer from the host to the guest, a host-guest exciplex, when formed, can provide a deactivation process, thus failing to cause luminescence in some cases. Accordingly, the selection of a host material determining an environmental condition surrounding a phosphorescent material is an important point for achieving an increased luminescence efficiency.